Geochemistry of Some Hp-Metavolcanics from Western Alps Metaophiolites

Acta Mineralogica-Petrographica, Szeged. XXX, 55—65,1989

GEOCHEMISTRY OF SOME HP-METAVOLCANICS FROM WESTERN ALPS METAOPHIOLITES

I. KUBOVICS and ABDHL AAL M. ABDEL-KARIM

Department of Petrology and Geochemistry, Lóránd Eötvös University

ABSTRACT

Ophiolite metavolvanics from Monviso, Arc valley and Montgenévre in the Piedmont Zone of the Western Alps are geochemically investigated and compared with data obtained from oceanic crust. Belonging to the Zermatt-Saas and Combin (Monviso and Arc valley) units they are characterized by HP-facies metamorphism including eclogite-facies metabasalts and garnet-bearing glaucophanites that underwent into greenschist-facies metamorphism. Few metavolcanics (Montgenévre) are partly escaped from ocean-floor metamorphism, but they are overprinted by Alpine deformation. They show a large variation in the bulk rock geochemistry and roughly compare to MORB and a few of them show an IAT character. Their geochemical characters prevalently similar to that of oceanic ridge basalts and they show abyssal tholeiitic differentiation trend. KEYWORDS: HP-Facies metavolcanics, Geochemistry, metamorphic evolution, Peidmont Zone, Western Alps metaophiolite.

INTRODUCTION *

The HP-metamorphism had an effect on large bodies of the pre-Alpine continental and organic crust in the Western Alps. During the Cretaceous, most of the oceanic crust (now represented by the Piedmont ophiolite nappe) and some parts of both continental margins underwent blueschist to eclogitic conditions that are followed by the greenschist facies conditions (LOMBARDO, 1988). The geochemistry of the metavolcanics from the Western Alps have been sub- jected to various studies ; previous researches had carried out from Monviso in Cottain Alps (LOMBARDO et al., 1978, NISIO, 1985, NISIO and LARDEAUX, 1987, COMPAGNONI et al., 1988), Arc valley in the Zermatt-Saas zone (BOCQUET, 1974, DAL PIAZ et al., 1981; DEN TEX, 1987; LEARDI et al., 1986) and from Chenaillet in Montgenévre (MÉVEL, 1975, LEWIS and SMEWING, 1980, BERTRAND et al., 1981, 1982, 1987).

FIELD RELATIONS

Field study and sampling of the metavolcanics were carried out in the eastern, central and western parts of the Piedmont ophiolite nappe in the Western Alps, e.g. Monviso, Arc valley area and Montgenevre ophiolites (Fig. 1.). The Piedmont ophiolite is represented by thinned, sheared, multistage folded and metamorphosed rem- nants of a narrow oceanic crust and related upper mantle (DAL PIAZ et al., 1981).

H-1088 Budapest, Muzeum krt. 4/a, Hungary.

55 Fig. 1. Tectonic sketch map of the internal Western Alps showing the location of the main ophiolite complexes. 1. Dora-Maire (DM) and Gran Paradiso (GP) continental units (European Paleomargin), 2. Vanoise, Ambin (AM) and Brianconnais (B), continental units (European Paleomargin) Mesozoic epicontinental covers, 4. Piedmont Zones (PZ): Schistes Lustres'nappe (Mesozoic, mainly oceanic material): a. undifferentiated metasediments with subordinate ophiolites; b. ophiolite complex with minor metasediments; c. metagabbro bodies. Location of samples: RA Refuged'A verole;PC=Pre clos la Clapera; CP-MC=Carrieres du Paradis-Mont Cenis; PB=Petit Belvedere; LL=Lago Lausetto; CF=Colleto Fiorenza; C=Chenaillet

56 Some of these metavolcanics are partly escaped from the strong Alpine deformation while others have no primary minerals of features. Belonging to the Combin and Zermatt-Saas Units they are in different structural and stratigraphic settings compared with other parts of the ophiolite sequences (peridotite, gabbros and metasediments). The overlying Combin unit displays pre-ophiolitic basal complex of Triassic to Liassic age with epicontinental affinity coverd by a thick volcanoclastic ophiolite bearing sequence (DAL PIAZ, 1974; DAL PIAZ et al, 1981; BEARTH, 1967; ETLER, 1971) which consists of metasediments and interbedded basaltic metavolcanics. This unit is also implied its relationship with other ophiolite sequences and characterized by an ocean-floor metamorphism which was overprinted by greenschist facies (HUN- ZIKER, 1974; DAL PIAZ et al. 1981). The underlying Zermatt-Saas unit consits of basal serpentinite, gabbro capped by retrograde eclogitic basalt and garnet-bearing metasediments (BEARTH, 1973; DAL PIAZ, 1974; DAL PIAZ et al., 1978, 1981). This unit has been deformed and affec- ted by HP-metamorphism and greenschist facies conditions and are considered as oceanic crust (BEARTH, 1967, DAL PIAZ, 1974, DAL PIAZ et al., 1978, 1981. e.t.c.). The Monviso metavolcanics show three successive Alpine metamorphic stages characterized by eclogitic, blueschist and greenschist facies. They include fine grained eclogitic metabasalts and banded metabasites (garnet-bearing glaucophanites and prasinites). From few cm-s up to several dm-s thick eclogitic metabasalts collected in Colletto Fiorenza, are crosscut by smaragditic metagabbros and younger albite

20 *

^ 15 o o tlaj. + + • ID- + • +V > T ¥ al- T Ax X * A + X 5 S' * io- vO m tf- 7,5- • V <3, 5- * + • + 2: 1- o 2,5- o AV + 70 VP V o + O*- 60 V •A •AA • * o *i • * + OCO? 50 * 40

0 05 0,6 0,7 0.8 0,9 0 0,6, Q7 OS 09 FeO+/ Fe0+' MgO ratio Fig. 2.: Major element oxide wt% versus FeO+ :eO++MgO ratio diagram for ophiolitic metavolcanics from Western Al i. (Symbols as in Table 1)

57 veins. The garnet-bearing glaucophanite in Petit Belvedere is usually heterogeneous and it shows banded layering. The prasinite (Lago Lausetto) represents a homogenous thin layer (mm-scale) which differs in colour and mineral composition. The Monviso metavolcanics appear to belong to the Zermatt-Saas unit. The Arc-valley metavolcanics of Combin unit consist of up to several hundered meters thick homogeneous prasinite which sometimes overlies on a thin ovardite level (the ovardite occurs as some thin interlations, too) and usually associate with garnet-bearing glaucophanite. They problably correspond to submarine flows, hyaloclastites and tuffites. The metavolcanic sequence of Montgenevre represents the best preserved ophiolite complex in the Western Alps (BERTRAND et al., 1987) and occurs as a separate tectonic unit. The pillow lavas are detached from the gabbros by a shear zone and sporadically contain serpentinite lenses.

PETROGRAPHY

The primary and secondary mineral assemblages, texture and metamorphic facies of the discussed metavolcanics are summarized in Table 1.

CHEMISTRY

Twelve selected metavolcanic samples collected from Monviso, Arc valley and Montgenevre in Western Alps were analysed. With the aim of the comparison we adapted six published analyses, as well. The location of samples are plotted in Fig. 1. and the chemical data appear in Table 2.

a) Analytical Methods

The silica was determined thermogravimetrically using the SAJO'S method <1955). The ferrous Fe was analysed by HOFFMANN'S method (301/87 OTH Patent). Ferric Fe was computed by the difference between FeO and total Fe, Al, total Fe, Mg, Ca, Na, K and Mn were measured by atomic absorption spectrophotometry. Other elements were determined by spectrophotometric method. H20, C02, and S02 were analysed by DTA method. The analyses were carried out by L. Hoffmann in the Department of Petrology and Geochemistry, Eötvös L. University, Budapest.

b) Results

The analysed HP-facies metavolcanics include 2 eclogitic metabasalts of Monviso (COMPAGNONI et al., 1988) and 3 garnet-bearing glaucophanites of Monviso and Arc valley (Table 2). The eclogitic metabasalts have large amounts of Si02 (50.61 */Al203/l 5.6 *) and Mno (0.48*) and small amounts of FeO+(8,47*) and Ti02 (1,4*). The garnet-bearing glaucophanite of Monviso differs from those of Arc valley in its much higher Ti02 (3.23*) and FeO+ (20.38*) and much lower A1203 (8.67*) and NaaO (3.65*) content, which may be ascribed to the abundance of Fe—Ti rich

58 TABLE 1 Main pétrographie features of representative rocks from Western Alps metavolcanics

Main primary minerals Main secondary minerals Rock name Texture (magmatic (hydrothermal and meta- Metamorphic fades and late-stage morphic) primary)

1. MONVISO: Coletto Fiorenza Eclogitic porph, gran cpx omp, gar, rut, clinoz, glau, Eclogitic f. metabas. Mg-chl, leuc, ab, phen, actin, Fe-chl,tit. Petit Belvedere Gar.-bearing granone- glau, leuc, ep, chl, ab, Glaucophane 5= glaucophanite matobl, stilp, actin, gar. schist f. O poikil, sch. Greenschist poikil, actin, chl, ab, tit, clinoz, Greenschist f. nematobl. cc. a Lago Lausetto o. Prasinite poikil, ab, chl, ep, glau, actin, Greenschist f. x phen, rut-tit.

2. ARC VALLEY Gar.-bearing granone- glau, ep, chl, ab, gar, tit, Glaucophane matobl, schist f. Glaucophanite poikil, sch. phen, cc, qz, bio. Prasinite poikil, sch. ab, ep. actin, rieb, glau, Greenschist f. chl, rut-tit, cc, phen, bio. Ovardite poikil, porp, ab, Fe-chl, glau, actin, Greenschist f. gne. ep, cc, rut-tit, phen, qz±gar.

3. MONTGENÉVRE: Chenaillet Metabas. pill, inters, porp, cpx, pi ab, chl, preh, ep, cc±zeol, lava aphy, que, pump. vario, arb, ves. Metabas. pill, inters, aphy, cpx, pi, epi, preh, chl, ab, hem± low-grade met.-ocean breccia vario pump. floor met. Metadol. lava vario, su- Prehn.-pump. and flow and dyke boph, interg, ol, cpx, pi, ab, chl, leuc, cc±preh greenschist facies Microgabbro hypid cpx, pi. hb. chl, actin-trem, leuc, ap,

Abbreviations: ol: olivine; cpx: clinopyroxene; pi: plagioclase; omp: omphacite; glau: glaucophane; actin: actionlite; trem: tremolite; rieb: riebeckite; hb: hornblende; ep: epidote; clinoz: clinozoisite; chl: chlorite; preh: prehnite; cc: calcite; ap: apatite; hem: hematite; leuc: leucoxene; rut: rutile, tit: titanite; bio: biotite; phen: phengite; stilp: stilpnomelane; gar: garnet; ab: albite; qz: quartz; pump: pumpellyite; zeo: zeolite; saus: saussurite; porp: porphyroblastic; poikil: poikiloblastic; nematobl: nematoblastic; granonematobl: granonematoblastic; gran: granular; hypid: hypidiomorphic, suboph: subophitic; inters: intersertal; interg: intergranular; sch: schistose, gne: gneissose; que: quench, vario: variolitic; arb: arborescent; ves: vesicular; metabas: metabasalt; pill: pillow; metadol: metadolerite; f: facies

59 Chemical composition of representative rock types from the ophiolitic

Arc valley Locality Carrières du Paradis Refuge Pre clos la Carrières du d'Averole • Clapera Paradis

Rock Prasinites Ovardites Glaucophanites name BOCGUET 1974 BOCGUET 1974

Symbols & Sample XI X2 X3 + 4 + 5 + 6 7(3) 08 9(4) • No.

Si02 47.94 50.95 51.81 52.13 44.55 42.75 49.80 53.05 51.46 Ti02 1.85 1.66 1.71 0.72 l.?3 1.41 1.43 0.90 1.84 AI2O3 15.95 15.74 14.92 14.72 13.01 11.23 16.88 16.9 14.68 Fe2Oa 3.80 3.89 0.95 0.92 7.54 2.01 5.78 10.49 5.48 FeO 7.89 6.56 9.04 6.34 2.70 8.84 4.57 2.92 6.58 MnO 0.17 0.16 0.15 0.14 0.13 0.13 0.17 0.09 0.20 MgO 4.96 5.28 5.55 5.42 5.01 9.92 5.90 2.44 5.76 CaO 8.07 3.99 4.45 5.87 11.96 11.85 6.27 5.16 6.06 Na20 4.12 5.32 3.57 5.39 4.32 2.56 4.90 6.02 4.45 K2O 0.70 0.23 1.73 0.10 0.15 0.82 0.48 0.93 0.80 + H2O 3.10 4.20 3.58 7.60 2.30 2.85 3.52 0.73 2.09 H2O- 0.3 0.60' 0.35 0.40 0.4 0.70 0.05 0.36 0.05 co2 0.7 1.60 ' 2.14 3.0 5.76 4.28 — — — P2O5 0.11 0.11 0.06 0.07 0.09 0.29 — 0.04 0.29 so2 0.10 — — — — — — — —

E 99.76 100.29 100.01 99.82 99.25 99.64 99.75 100.03 99.74 n.d.: non detected ( ): number of analyses oxide. As these chemical features are also recognized in some Fe—Ti rich gabbros observed in the same area (in press) and in the others within the Alpine-Apennine belt (BACCALUVA et al., 1977, LOMBERDO et al., 1978, 1982, DAL PIAZ et al., 1981, BERTRAND et al., 1987), we can expect that both the glaucophanites and Fe—Ti gabbros were probably derived from the same magma. The analysed greenschist facies metavolcanics are as follows: 1 greenschist (Monviso), 4 prasinites (Monviso and Arc valley) and 4 ovardites (Arc valley). + The greenschist has a higher MgO (9.15*) and lower Ti02 (0,62*) and FeO (9.83*) contents. The prasinite from Monviso is characterized by a higher A1203 (19.01*), MgO (6.01*) and K20 (3,46*) and a lower Ti02 (1,4*) content than that of the samples collected in the Arc valley. The higher values of A1203 and MgO may be attributed to the diluation of plagioclase and olivine accumulation. The high Ti02 content may due to the abundant phengite. The prasinites (from Arc) cover a wide compositional ranges of SiOa (47.94—51.8*), FeO+ (9.99—11.69*), CaO (3.99—8.07*) and C02 (0,7—2.14*) and also those elements whitch are relatively .stable during the altera- tion, such as P2Oa (0.06—0.11*) and Ti02 (1.66—1.85*). Their higher Al2Os

60 TABLE 2 volcanic sequence of Western Alps

Mon viso Montgenévre

Petit Coletto Lago Chenaillet Belvedere Fiorenza Lausetto Metadolerite Green- eclogitic Prasinite Basaltic pill, lavas schist metabas. flow and dyke COMPAGNONI et al., BERTRAND BERTRAND et al., 1987 1988. et al., 1987

• 10 All •12(2) kl3 VL4 15(9)V r 16 T17 T18(11)

50.05 50.63 50.61 47.62 57.00 51.25 56.20 56.43 50.16 3.23 0.62 1.40 1.40 0.81 1.68 1.25 1.29 1.62 8.67 13.36 15.60 19.01 13.52 15.35 10.41 11.64 15.79 12.52 2.97 1.40 10.46 1.57 2.59 1.94 2.92 2.43 7.84 6.42 7.07 n.d. 5.53 6.00 6.28 5.66 6.13 0.23 0.11 0.48 0.10 0.10 0.19 0.14 0.16 0.15 5.33 9.15 7.44 6.01 6.32 6.18 9.05 5.96 5.95 6.76 6.63 10.63 3.51 5.91 7.87 6.83 8.33 8.33 3.65 4.62 3.31 5.00 5.85 4.55 4.81 5.16 4.95 0.10 0.11 0.27 3.46 0.10 0.11 0.10 0.10 0.04 0.90 3.40 1.33 2.76 2.60 1.40 3.51 . 0.20 1.00 1.07 2.9 ' 1.00 0.60 0.20 0.95 0.61 0.03 0.09 0.34 0.12 0.06 0.23 0.23 0.11 0.20 0.15

99.51 99.11 99.62 99.59 99.00 99.70 100.34 99.26 99.87

(14.72—15.95*) and CaO (3.99—8.07*) contents may attributed to the diluation of the plagioclase. The Arc valley ovardites also show largely variable SiOa (42.75—52.13*), FeO+ (7.26—10.85*), CaO (5.87—11.96*), P205 (0.07—0.29*) and TiOz (0.72—1,43*) contents. The ovardite at Pre clos la Clapera has a significantly higher MgO content (9.92%) than the other ovardites of the Arc area, which suggest their strong dilution effect by olivine accumulation or fractionation. The chemical features of both prasinites and ovardites are well compared, however, revealed certain significant differences themselves to be excepted in view of the ovardites. MgO, CaO, COa contents are higher, while Ti02 content is lower than the mean for prasinites. This is probably ascribable to the particulary high chlorite and calcite content in the ovardites. The analysed samples from Mongenevre represent 1 metadolerite pillow and 2 basaltic pillow lavas (margin and core) compared with their averages (BERTRAND et ai., 1987). The metadolerite pillow flow is characterized by higher values of SiOa (57.0*) and Na20 (5.58*) and by lower values of Ti02 (0.81*), FeO* (7.71*), A1203 (13.52*) and P206 (0.14*), than the average given by BERTRAND et al. (1987).

61 The basaltic pillow lavas have a significantly higher Si02 (56.2—56.43*) and lower TiOz (1.25—1.29*) and A1203 (10.41—11.64*) range than the average (BERT- RAND etal. 1987): - . It is interesting that the pillow lava margin has fairly higher MgO and H20 and lower A1203, total FeO, CaO and Na20 contents than the core has. All these geochemical features can be attributed probably to the variant influence of the metasomatic metamorphism.

DISCUSSION

Major element composition (Table 1.) and their variation againts FeO+/FeO+ + +MgO ratio of the metavolcanic rocks from Western Alps are shown in Fig. 2. Generally, the total FeO* and TiOs content are rapidly increased, Na20* is constant + while the MgO*, P20£ and Si02 are decreased with the increasing FeO +MgO ratio. The garnet-bearing glaucophanites are largely scattered which may be attributed to their different tectonic setting and mineralogic compositions. The AFM diagram (Fig. 3.) applied by STRONG and MALPAS (1975) represents a tholeiitic differentiation. Some of the investigated metavolcanic rocks are tholeiitic character, while the others show calc-alkaline affinity because of their metamorphic effects and mixing with oceanic floor sediments. They mostly fall within the sheeted dyke and lava fields of the STRONG'S and MALPAS'S (1975) diagram. The garnet- bearing glaucophanites from Monviso, due to its higher content of total FeO which can be attributed to the dominance of the Fe-rich oxides, occur near the F-pole. Two samples of prasinites and one of dolerite flow have a more alkalic charcater than the others, indicating a more significant influence of the metamorphism.

Fig. 3. AFM diagram (STRONG and MALPAS, 1975) for the ophiolitic metavolcanics from Western Alps. (Symbols as in Table 2.)

*: weight percentage FeOFeO1+Fe,Oi - -

62 The Ti02 vs. P205 and the TiOz vs. FeO+/MgO diagrams applied by BASS et al. (1973) and HEKINIAN and THOMPSON (1976), present the differentitation trend of the ophiolitic basaltic rocks (Fig. 4.). In the diagram of Ti02 vs. P205, the metavolcanic rocks are situated in the oceanic ridge basalt field showing a constant precipitation of apatite during the volcanic differentation. In the Ti05 v. FeO+/MgO diagram, the metavolcanics more or less correspond to the trend of the abyssal tholeiites showing an increasing Ti02 content, together with the increasing FeO*/MgO ratio. The garnet-bearing glaucophanite from Monviso in it higher content of Ti02

Fig. 4. (a) P2Oe wt% versus TiOswt% diagram (HEKINIAN et al. 1976) and (b) Ti02 WT% versus FeO* /MgO ratio diagram (BASS et al. 1973) for the ophiolitic metavolcanics from Western Alps. (Symbols as in Table 2.)

Fig. 5. MnOX 10—TiOj—PjOsX 10 diagram (MULLEN, 1983) for the ophiolitic metavolcanics from Western Alps. MORD=mid ocean ridge and marginal basin basalts, IAT= island arc tholeiites, CAB=calc-alkaline basalts, OIA=oceanic island alkalic basalts. (Symbols as in Table 2.)

63 and total FeO (which can be attributed to the abundance of Ti—Fe rich oxides) differs from that of the Arc valley rocks.

The Ti02—MNOX 10—P2OSX 10 diagram (Fig. 5.) was initiated by MULLEN (1983) for the distinction of the tectonic settings of the basaltic and andesitic rocks. In this diagram, the metavolcanics of the Western Alps are almost situated in the MORB and Island Arc Tholeiites fields.

CONCLUSIONS

The Western Alps metavolcanics are distributed in both Combin and Zermatt- Saas units of the Piedmont metaophiolite. Three metavolcanic areas (Monviso, Arc valley and Montgenévre) in the Western Alps were investigated geochemicaly. The metavolcanics of the Monviso and Montgenévre in the Zermatt-Saas unit show mostly oceanic crust origin. The Arc valley metavolcanics in the Combin unit are represented by basaltic metabasites (prasinites, ovardites and glaucophanites) and most of them probably consist of lava flows, hyaloclastites. tuffites and minor sills settled in Mesozoic calc-schists (DAL PIAZ, 1974). The investigated metavolcanics from Western Alps, due to the significant changes in the mineral abundances, show a large variation in the bulk composition. This probably reflects an effect of the Alpine and sea-floor metamorphism. They are roughly comparable to MORB (HEKINIAN and THOMPSON, 1976; SUN et al., 1979; WOOD et al., 1979 b; LANGMUIR and BENDER, 1984), and a few metavolcanics belong to the Island Arc Tholeiites (as shown in MULLEN'S 1983 diagram). They also show a geochemical characteristic prevalently similar to that of the oceanic ridge1 basalts {BASS et al. 1973) and differentiation path typical for abyssal tholeiites (HEKINIAN etal. 1976). Morever, evidence for happened, distrinctive metamorphic and tectonic processes the oceanic stage can be recognized (MÉVEL et al. 1978; BERTRAND et al. 1985; TRICART and LEMOINE 1986).

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Manuscript received, 30 May, 1989